D-alanyl-D-alanine peptidases catalyze the crosslinking of peptidoglycan in the final stages of bacterial cell wall biosynthesis. The DD-peptidases are the targets for B-lactam antiobiotics (penicillins and cephalosporins). The structure of the enzyme from Streptomyces R61 has been determined to 1.6A resolution (Crystallographic R-factor = 0.153). We are now studying complexes of the native enzyme with drug molecules and substrate analogs. These studies will be facilitated by access to the Cambridge Structural Database.